As much as half of the gas mass in our galaxy's dark matter halo may reside not in the galaxy itself, but in the surrounding area, the circumgalactic medium (CGM). The vast gas content of the CGM, loosely defined as the volume immediately outside the galaxy but inside the dark matter halo, can be broadly classified as originating either in accretion from the intergalactic medium (IGM) or winds from galaxies. Both of these are crucial to the process of galaxy formation: IGM accretion provides the material necessary for observed star formation rates, while galactic winds are linked closely to the regulation of star formation through feedback. Despite their individual importance, differentiating these gas flows in observations is an outstanding problem in studies of the CGM. I will discuss our efforts to address this problem through the use of hydrodynamic galaxy formation simulations on two fronts: mock observations of CGM quasar absorption lines and "particle-tracking" analyses that reconstruct the full history of CGM gas.

 Galaxy clusters are the largest virialized objects in the Universe, and are powerful probes of cosmology. Their abundance as a function of mass and redshift is extremely sensitive to how structures grow and the properties of dark energy. Though they are powerful probes of cosmology, they are currently limited by mass uncertainty which is ~ 15%. One way to measure cluster mass is through lensing of the cosmic microwave background (CMB). CMB-cluster lensing offers a robust and accurate way to constrain galaxy cluster masses, especially at high redshift (z > 1) where optical lensing measurements are challenging. With CMB lensing we expect to improve mass uncertainty to 3% for upcoming experiments such as AdVACT, SPT-3G etc., and to 1% for next-generation CMB experiments. In this talk, I will report on a demonstration of this technique using CMB data from the South Pole Telescope (SPT) to constrain masses of the Dark Energy Survey (DES) galaxy cluster catalog.